Paper feeding unit for preventing multiple feeding

ABSTRACT

A paper feeding unit for substantially preventing multiple feeding is provided. The paper feeding unit includes a paper feeding cassette that stores printing mediums, and a knock-up plate rotatably disposed in the paper feeding cassette and on which the printing mediums are loaded. A knock-up spring applies an elastic force to the knock-up plate. A protruding portion, which is formed by protruding the knock-up plate, has a friction force with the printing medium loaded thereon greater than the friction force between printing mediums. Accordingly, multiple feeding is substantially prevented, and the manufacturing and assembly costs are reduced.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2005-0082631, filed on Sep. 6, 2005, in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a paper feeding unit. More particularly, the present invention relates to a paper feeding unit capable of substantially preventing a printing medium from multiple feeding.

2. Description of the Related Art

Generally, image forming apparatuses, such as printers or copy machines, include a paper feeding unit wherein printing mediums, such as papers or overhead projection (OHP) films, are stored and the stored printing mediums are sequentially supplied into the main frame of the image forming apparatus. FIG. 1 is a perspective view of a conventional paper feeding unit having a paper feeding cassette 100, a knock-up plate 120, a knock-up spring 130, a friction pad 150, a hinge boss 101, and a hinge notch 121. The paper feeding cassette 100 for storing a printing medium is detachably connected to the image forming apparatus. The knock-up plate 120 is rotatably disposed in the paper feeding cassette 100. The printing medium is loaded on the upper part thereof, and the lower part thereof is elastically supported by the knock-up spring 130. The knock-up plate 120 rotates in the paper feeding cassette 100 according to the amount of the loaded printing medium. The hinge boss 101, which is provided at the paper feeding cassette 100, and the hinge notch 121, which is provided at one end of the knock-up plate 120 so that the hinge boss 101 is rotatably inserted, become the rotation center of the knock-up plate 120.

The printing medium loaded on the knock-up plate 120 must be supplied to the main frame of the image forming apparatus sheet by sheet. The printing mediums may be jammed when more than two printing mediums are overlapped so that multiple feeding takes place. As a result, the image forming apparatus may operate inappropriately or stop operating. Although not shown, an element for preventing multiple feeding is provided. A common element for preventing multiple feeding can be implemented in various forms.

Particularly, the friction pad 150 is provided to prevent multiple feeding when few printing mediums are loaded. Two friction pads 150 are respectively provided at both ends of the knock-up plate 120 facing a pick-up roller. The friction pad 150 is made of foamable urethane or cork. Considering the width of the pick-up roller, the width of the friction pad 150 must be wide enough to provide sufficient friction force required to prevent multiple feeding. The friction pad 150 must apply the same friction force onto the printing medium at the left and right sides of the knock-up plate 120. The friction pad 150 is attached at the position symmetrical to both ends of the knock-up plate 120 by adhesive materials. If the width and length of the friction pads 150, attachment direction, or friction coefficient are different from each other, the front edge portion of the feeding printing medium may be skewed. Thus, the friction pads 150 respectively attached at the left and right sides of the knock-up plate 120 must have the same characteristics. Accordingly, the manufacturing and assembly costs of the friction pads 150 are high.

Accordingly, a need exists for an improved feeding unit of an image forming apparatus that substantially prevents multiple feeding of printing media.

SUMMARY OF THE INVENTION

The exemplary embodiments of the present invention provide a paper feeding unit that substantially prevents multiple feeding and is easily assembled and maintained.

According to an aspect of the present invention, a paper feeding unit comprises a paper feeding cassette that stores printing mediums to be supplied to a main frame of an image forming apparatus by a pick-up roller, and a knock-up plate that is rotatably disposed in the paper feeding cassette and on which the printing mediums are loaded. A knock-up spring applies an elastic force to the knock-up plate towards the pick-up roller. Protruding portions are formed by protruding the knock-up plate at a position facing the pick-up roller and of which friction force with the printing medium loaded thereon is greater than the friction force between the printing mediums.

In an aspect of the paper feeding unit, the protruding portions may be formed by a blanking process of the knock-up plate.

According to another aspect of the present invention, a paper feeding unit comprises a paper feeding cassette that stores printing mediums to be supplied to the main frame of an image forming apparatus by a pick-up roller, and a knock-up plate that is rotatably disposed in the paper feeding cassette and on which the printing mediums are loaded. A knock-up spring applies an elastic force to the knock-up plate towards the pick-up roller. A protruding portion is provided to the knock-up plate using an outsert molding method at a position facing the pick-up roller and of which friction force with the printing medium loaded thereon is greater than the friction force between the printing mediums.

In an aspect of the paper feeding unit, the protruding portion may protrude less than 0.2 mm with respect to the surface of the knock-up plate.

Additionally, the protruding portion may be formed in a portion of the knock-up plate that is pressed in the opposite direction with respect to the pick-up roller.

Additionally, the protruding portion may be right-left symmetrical along with a pick-up direction of the printing medium.

Additionally, the surface of the protruding portion may be subject to an embossing process.

Additionally, the protruding portion may include one or more squares having a hole in the center thereof.

Additionally, at least one of the squares has a hole in the center thereof inclined toward a pick-up direction of the printing medium.

Other objects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:

FIG. 1 is a perspective view of a conventional paper feeding unit having a friction pad;

FIG. 2 is an elevational view in cross section of an image forming apparatus including a paper feeding unit according to an exemplary embodiment of the present invention;

FIG. 3 is an elevational view in cross section illustrating the operation of a paper feeding unit according to an exemplary embodiment of the present invention;

FIG. 4 is an elevational view in cross section illustrating the operation of a protruding portion according to an exemplary embodiment of the present invention; and

FIG. 5 is a perspective view of a knock-up plate according to an exemplary embodiment of the present invention.

Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings.

FIG. 2 is an elevational view in cross section of an image forming apparatus 1 according to an exemplary embodiment of the present invention. In the present exemplary embodiment, the image forming apparatus 1 is an electrophotographic image forming apparatus. The electrophotographic image forming apparatus 1 includes a main frame 4 of the image forming apparatus, a developing unit 20 provided in the main frame 4 of the image forming apparatus, a photoconductor 30, and a paper feeding cassette 400.

A charging bias voltage is applied to a charging roller 39 so that the outer circumference of the photoconductor 30 is equipotentially charged. The outer circumference of the photoconductor 30 may be uniformly charged with a negative voltage of hundreds of volts by a negative charging bias voltage. An exposure unit 10 illuminates light corresponding to image data on the photoconductor 30 according to computer signals to form an electrostatic latent image on the outer circumferential surface of the photoconductor 30. Resistances are reduced on the portions where light is illuminated, so the electrostatic latent image has a negative voltage less than tens of volts as negative charges are discharged. The exposure unit 10 includes a light source (not shown) and a deflector 12 for deflecting light illuminated by the light source.

The developing unit 20, a cartridge that is detachably disposed in the main frame 4 of the image forming apparatus, stores toners and includes a housing 22 forming the enclosure of the developing unit 20, wherein a developing roller 40, a toner layer control unit 50, a supply roller 60, and an agitator 62 are provided. A toner storage 25 for storing solid powder type dry toners as a developing material is provided in the housing 22. The developing unit 20 is replaced when the toners stored in the toner storage 25 are used up. A door 3 is provided at the side of the main frame 4 of the image forming apparatus so that the developing unit 20 may be detached from the main frame 4 of the image forming apparatus.

The developing roller 40 attaches the toners stored in the housing 22 on the outer circumference thereof to be supplied to the photoconductor 30. The developing roller 40 supplies the toners attached on the outer circumference thereof to the electrostatic latent image formed on the photoconductor 30 and carries out developing. The electrostatic latent image is developed in a developing area where the photoconductor 30 and the developing roller 40 face each other. A developing nip or a developing gap is generally referred to as the developing area. A developing bias voltage is applied to the developing roller 40 so that the toners are transferred to the photoconductor 30 to be attached thereon. The developing bias voltage may have a waveform wherein a certain alternating current voltage is overlapped on a direct current voltage. The toners are commonly charged with a negative voltage. Due to the developing bias voltage having the aforementioned waveform, the toners transferred through the developing area are attached on the electrostatic latent image having a negative voltage less than tens of volts and cannot be attached on a non-image area where the charging bias voltage is applied. When developing using a non-contact jump method, the electrostatic latent image is formed while the toners having a negative voltage are vibrated and reciprocated in a developing gap b.

The supply roller 60 supplies the toners to the developing roller 40. The agitator 62 agitates the toners in the toner storage 25 so that the toners are not hardened and transfers the toners toward the supply roller 60.

The toner layer control unit 50, wherein one side thereof is fixed to the housing 22 and the other side thereof is in contact with the developing roller 40, controls the height of the toners attached on the outer circumferential surface of the developing roller 40 and frictionally charges the toners to a predetermined polarity (for example, a negative voltage). The toner layer control unit 50 is a metal plate and elastically contacts the outer circumference of the developing roller 40.

A free end portion a is formed on the other side of the toner layer control unit 50. The thickness of the controlled toner layer is determined by the length from a contact position of the toner layer control unit 50 and the developing roller 40 to the end edge of the free end portion a.

A photoconductive material layer is coated on the outer circumferential surface of the photoconductor 30, a cylindrical metal drum, by a method such as deposition, and a portion of the outer circumferential surface thereof is exposed towards the printing medium P. The exposed outer circumferential surface of the photoconductor 30 faces a transfer roller 70.

A cleaning member 38 is disposed in a waste-toner container 32. The cleaning member 38 contacts the photoconductor 30 by a predetermined pressure so that toner residuals from the photoconductor 30 may be scraped after the toner transfer is ended. The waste-toner container 32 includes a waste-toner storage 23. Toners separated from the photoconductor 30 by the cleaning member 38 are stored in the waste-toner storage 23.

The transfer roller 70 faces the outer circumferential surface of the photoconductor 30. The transfer roller 70 transfers a toner image on the outer circumferential surface of the photoconductor 30 to the printing medium P. A transfer bias voltage having opposite polarity with respect to the toner image is applied to the transfer roller 70. The toner image is transferred onto the printing medium P by an electrostatic force acting between the photoconductor 30 and the developing roller 40.

A fixing unit 75, which includes a heating roller 76 and a pressure roller 77, fuses the toner image onto the printing medium P by applying heat and pressure onto the toner image transferred on the printing medium P. The heating roller 76 includes a heating coil or a halogen lamp as a heat source for permanently fixing the toner image. The pressure roller 77 presses the printing medium P passing through a fusing nip.

A decurl unit 78 removes curls on the printing medium P generated by heat and pressure of the fixing unit 75. After fusing, an out-feed roller 79 feeds the printing medium P out of the image forming apparatus 1. The printing medium P fed out of the image forming apparatus 1 is loaded on a paper feeding tray 2.

The paper feeding cassette 400 stores the printing medium P. A knock-up plate 420, which is elastically supported by a knock-up spring 430 and on which the printing medium P is loaded, is rotatably provided in the paper feeding cassette 400. A pick-up roller 480 picks up the printing medium P loaded on the knock-up plate 420 sheet by sheet to be fed out. A feed roller 81 feeds the picked up printing medium P toward an arranging unit 90. The arranging unit 90 arranges the front edge portion of the printing medium P before the printing medium P passes the contact surface of the photoconductor 30 and the transfer roller 70, so that the toner image may be transferred onto a desired portion on the printing medium P.

Although not shown, a plurality of developing cartridges are needed for color printing using an electrophotographic method. For example, an image forming apparatus using a multi-pass method includes one photoconductor and four developing cartridges. In this method, four developing cartridges corresponding to cyan C, magenta M, yellow Y, and black K are provided, and four cycles of exposing, photo-sensing, and developing processes for respective colors are sequentially performed, so that a whole color image is printed. Alternatively, an image forming apparatus using a single-pass method includes four developing cartridges corresponding to the four colors and four photoconductors. In this method, the exposing, photo-sensing, and developing processes for the respective colors are performed concurrently, so that fast printing speed is realized. An image forming apparatus using a two-pass method, in which the multi-pass method and the single method are combined, includes two unit elements, and a unit element is composed of two developing cartridges and one photoconductor.

FIG. 3 is an elevational view in cross section illustrating the operation of a paper feeding unit according to an exemplary embodiment of the present invention. FIG. 4 is an elevational view in cross section illustrating the operation of a protruding portion according to an exemplary embodiment of the present invention. FIG. 5 is a perspective view of a knock-up plate according to an exemplary embodiment of the present invention. Referring to FIGS. 3 through 5, protruding portions 500 a and 500 b are provided for substantially preventing a paper feeding unit from multiple feeding. The x-axis is the direction in which a printing medium P is picked up. The paper feeding unit includes a paper feeding cassette 400, a knock-up plate 420, and a knock-up spring 430. The paper feeding cassette 400 is detachably connected to the main frame 4 of the image forming apparatus. The paper feeding cassette 400 is separated from the main frame 4 of the image forming apparatus when the printing medium P stored therein is used up and placed in the main frame 4 of the image forming apparatus again after the printing medium P is supplemented.

The knock-up plate 420 is provided in the paper feeding cassette 400. The knock-up plate 420 has a shape of a wide flat plane having a size of the printing medium P so that the printing medium P is loaded on the upper end thereof, and the lower end thereof is elastically supported by the knock-up spring 430. A hinge notch 421 formed at one end of the knock-up plate 420 is rotatably connected with a hinge boss 401 of the paper feeding cassette 400. Although not shown, in another exemplary embodiment, the hinge notch 421 may be connected to the paper feeding cassette 400, and the hinge boss 401 may be connected to the knock-up plate 420. The other end of the knock-up plate 420 is elastically biased towards a pick-up roller 480 by the knock-up spring 430. The knock-up plate 420 rotates towards the pick-up roller 480 when the number of the loaded printing medium P is reduced.

The pick-up roller 480 is disposed in the main frame 4 of the image forming apparatus and is connected to a rotation axis 490 thereof. The pick-up roller 480 picks up the printing medium P sheet by sheet while being rotated by a pick-up roller driving element (not shown). The pick-up roller 480 has a long radius portion 481 in contact with the printing medium P and a short radius portion separated from the printing medium P. The outer circumferential surface of the long radius portion 481 has a sufficient friction coefficient for picking up the printing medium P. The friction coefficient between the long radius portion 481 and the printing medium P is greater than the friction coefficient between printing mediums P1 and P2. Therefore, when the outer circumferential surface of the long radius portion 481 comes in contact with the printing medium P, only a sheet of printing medium P is picked up.

Although not shown, a finger member may be further provided for picking up the printing medium P sheet by sheet. The finger member partially obstructs feeding of the printing medium P by holding both ends of the front edge portion of the printing medium P, such that only the printing medium P in contact with the pick-up roller 480 is picked up in the x-axis direction. The finger member obstructs feeding even when a friction force is generated between the printing medium P picked up and the printing medium P beneath, so that only one printing medium P is fed. Due to the feeding obstruction by the finger member, curl is generated at both ends of the front edge portion of the feeding printing medium P. To substantially prevent jamming of the printing medium P, the finger member has such a shape and size that curl generation may be controlled within a predetermined range.

The long radius portion 481 comes in contact with the printing medium P while the pick-up roller 480 is rotating. As the knock-up plate 420, which is elastically supported by the knock-up spring 430, and the printing medium P are pushed downwards, the printing medium P is picked up. When the pick-up roller 480 continuously rotates, the long radius portion 481 is separated from the printing medium P, and then the top edge portion of the printing medium P faces the short radius portion. The knock-up plate 420 and the printing medium P are lifted up to a certain height and then enter into a stand-by mode for a next pick-up operation. The height of the loaded printing medium P is substantially constantly maintained even after one sheet of the printing medium P is picked up by the knock-up plate 420. Although not shown, the height of the printing medium P to be picked up is controlled to be substantially constant by an additional stopper or finger member.

A pick-up force of the pick-up roller 480 is the friction force acting between the long radius portion 481 and the printing medium P. The friction force is the product of a friction coefficient and a normal force. The normal force perpendicularly acts on a pick-up surface (parallel with the inclined knock-up plate 420). A feeding angle θ is defined as the angle between the x-axis, parallel with the bottom surface of the paper feeding cassette 400, and the inclined knock-up plate 420. The normal force is a component of the elastic force generated by the knock-up spring 430 perpendicularly acting on the pick-up surface. Specifically, the normal force is a component of the resultant force acting in the y-axis direction, that is, a component of the force obtained by subtracting the weight of the printing medium P from the elastic force generated by the knock-up spring 430. For convenience, the normal force is approximated to a component force of the elastic force generated by the knock-up spring 430, and the weight of the printing medium P is ignored. Thus, the normal force is the product of the elastic force generated by the knock-up spring 430 and cos θ, where θ is the feeding angle. Assuming that the elastic force generated by the knock-up spring 430 and the friction coefficient are constant, the pick-up force of the pick-up roller 480 decreases as the feeding angle θ increases. Namely, when ignoring the weight of the printing medium P, the pick-up force decreases while the printing medium P is being picked up and the rotation angle of the knock-up plate 420 increases.

FIG. 4 is an elevational view in cross section illustrating the operation of protruding portions 500 a and 500 b for substantially preventing multiple feeding when two sheets of printing mediums P1 and P2 are loaded on the knock-up plate 420. Here, the feeding angle θ is considered to have a maximum value, and the pick-up force is considered to have a minimum value. As described above, the pick-up force is the friction force acting between the long radius portion 481 of the pick-up roller 480 and the printing medium P1. To facilitate the pick-up process, the pick-up force is significantly greater than the friction force acting between the two printing mediums P1 and P2. To substantially prevent multiple feeding, the friction force acting between the printing medium P2 and the protruding portions 500 a and 500 b is greater than the friction force acting between the two printing mediums P1 and P2. The magnitudes of these forces are in the following relationship: Pick-up force>>Friction force acting between the printing medium P2 and the protruding portions 500 a and 500 b>Friction force acting between the two printing mediums P1 and P2  [Inequality 1]

In an exemplary embodiment of the present invention, the protruding portions 500 a and 500 b are formed by protruding the knock-up plate 420 in a position facing the pick-up roller 480. A blanking process is carried out on the protruding portions 500 a and 500 b so that a portion of the knock-up plate 420 is protruded towards the pick-up roller 480. The friction force between the printing medium P2 and the protruding portions 500 a and 500 b is greater than the friction force between the printing medium P1 and the printing medium. P2.

In another exemplary embodiment of the present invention, the protruding portions 500 a and 500 b are provided to the knock-up plate 420 in a position facing the pick-up roller 480 using an outsert molding method. The protruding portions 500 a and 500 b are made of a material different from that of the knock-up plate 420 and are incorporated on the knock-up plate 420 by the outsert molding method. The friction force between the printing medium P2 and the protruding portions 500 a and 500 b is greater than the friction force between the printing medium P1 and the printing medium P2. The blanking process and the outsert molding method is not described in detail. The number of processes and the manufacturing costs required to attach the conventional friction pad 150 of FIG. 1 are reduced when using the protruding portions 500 a and 500 b formed by protruding a portion of the knock-up plate 420 or by outsert molding the knock-up plate 420 using a material having a large friction coefficient, instead of the conventional friction pad 150 of FIG. 1.

The printing medium P may be detached from the surface of the knock-up plate 420 as the protrusion degree, or height, “t” of the protruding portions 500 a and 500 b increases, so the printing medium P may not be properly fed. In an exemplary embodiment of the present invention, the protrusion degree “t” of the protruding portions 500 a and 500 b is controlled to be less than about 0.2 mm. The protruding portions 500 a and 500 b may be directly formed on the knock-up plate 420 in flat condition. Sometimes it is difficult to control the protrusion degree “t” to be less than about 0.2 mm in a manufacturing process, Such that in an exemplary embodiment of the present invention a portion of the knock-up plate 420 is depressed in the opposite direction with respect to the pick-up roller 480 to form a depressing portion 520. Once the protruding portions 500 a and 500 b are formed on the depressing portion 520, the protrusion degree “t” with respect to the knock-up plate 420 may be controlled to be less than about 0.2 mm even though the height of the protruding portions 500 a and 500 b with respect to the surface of the depressing portion 520 becomes more than 0.2 mm. This is because the depth of depression of the depressing portion 520 and the height of the protruding portions 500 a and 500 b exceeding 0.2 mm compensate each other. Additionally, the depressing portion 520 allows the long radius portion 481 of the pick-up roller 480 to face only the protruding portions 500 a and 500 b, thereby increasing the friction force of the printing medium P2 with respect to the protruding portions 500 a and 500 b.

When the protruding portions 500 a and 500 b are formed using a blanking process, the protruding portions 500 a and 500 b are made of the same material of the knock-up plate 420. When the protruding portions 500 a and 500 b are formed using an outsert molding process, the protruding portions 500 a and 500 b are preferably made of rubber or synthetic resin that provides the aforementioned predetermined friction force.

The protruding portions 500 a and 500 b are arranged on left and right sides of the knock-up plate 420 with respect to the pick-up direction of the printing medium P. The protruding portions 500 a and 500 b on the left side of the knock-up plate 420 are symmetrical to those on the right side thereof to substantially prevent the left and right sides of the front edge portion of the printing medium P from being skewed. In an exemplary embodiment of the present invention, the surface of the protruding portions 500 a and 500 b undergoes an embossing process to increase the friction coefficient.

The protruding portions 500 a and 500 b preferably include one or more squares 510 having a hole in the center thereof. By removing the center portion of the square, the peripheral portion of the square may be easily protruded. As shown by reference 500 a in FIG. 5, the square is preferably disposed parallel to each of the x and y axis directions in order to increase the friction coefficient in each of the x and y axis directions. As shown by reference 500 b in FIG. 5, at least one of the squares is disposed to be inclined in the pick-up position, such that the friction coefficient is increased in the inclining direction and the perpendicular direction thereto.

Accordingly, in a feeding unit of the present invention, multiple feeding of a printing medium may be substantially prevented by providing a protruding portion having a friction force greater than that between printing mediums and a knock-up plate. Also, manufacturing and assembly costs are reduced by protruding the knock-up plate using a blanking process or by providing protruding portions using an outsert molding.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims. 

1. A paper feeding unit, comprising: a paper feeding cassette that stores printing mediums to be supplied by a pick-up roller to a main frame of an image forming apparatus; a knock-up plate rotatably disposed in the paper feeding cassette and on which the printing mediums are loaded; a knock-up spring that applies an elastic force to the knock-up plate toward the pick-up roller; and a protruding portion formed by protruding the knock-up plate at a position facing the pick-up roller and of which friction force with the printing medium loaded thereon is greater than the friction force between the printing mediums.
 2. The paper feeding unit according to claim 1, wherein the protruding portion is formed by a blanking process of the knock-up plate.
 3. The paper feeding unit according to claim 2, wherein the protruding portion has a protrusion degree less than about 0.2 mm with respect to the surface of the knock-up plate.
 4. The paper feeding unit according to claim 3, wherein the protruding portion is formed in a portion of the knock-up plate that is depressed in the opposite direction with respect to the pick-up roller.
 5. The paper feeding unit according to claim 1, wherein at least two protruding portions are arranged on left and right sides of the knock-up plate with respect to a pick-up direction of the printing medium so that a configuration of the protruding portions on the left side is symmetrical to a configuration of the protruding portions on the right side.
 6. The paper feeding unit according to claim 1, wherein the surface of the protruding portion is subject to an embossing process.
 7. The paper feeding unit according to claim 1, wherein the protruding portion includes one or more squares having a hole in the center thereof.
 8. The paper feeding unit according to claim 7, wherein at least one of the squares having a hole in the center thereof is inclined with respect to a pick-up direction of the printing medium.
 9. A paper feeding unit, comprising: a paper feeding cassette that stores printing mediums to be supplied by a pick-up roller to the main frame of an image forming apparatus; a knock-up plate that is rotatably disposed in the paper feeding cassette and on which the printing mediums are loaded; a knock-up spring that applies an elastic force to the knock-up plate towards the pick-up roller; and a protruding portion disposed on the knock-up plate using an outsert molding method at a position facing the pick-up roller and of which a friction force with the printing medium loaded thereon is greater than the friction force between the printing mediums.
 10. The paper feeding unit according to claim 9, wherein the protruding portion has a protrusion degree less than about 0.2 mm with respect to the surface of the knock-up plate.
 11. The paper feeding unit according to claim 10, wherein the protruding portion is formed in a portion of the knock-up plate that is depressed in the opposite direction with respect to the pick-up roller.
 12. The paper feeding unit according to claim 9, wherein at least two protruding portions are arranged on left and right sides of the knock-up plate with respect to a pick-up direction of the printing medium so that a configuration of the protruding portions on the left side is symmetrical to a configuration of the protruding portions on the right side.
 13. The paper feeding unit according to claim 9, wherein the surface of the protruding portion is subject to an embossing process.
 14. The paper feeding unit according to claim 9, wherein the protruding portion includes one or more squares having a hole in the center thereof.
 15. The paper feeding unit according to claim 14, wherein at least one of the squares having a hole in the center thereof is inclined with respect to a pick-up direction of the printing medium.
 16. A paper feeding unit, comprising: a paper feeding cassette storing printing mediums to be supplied by a pick-up roller to a main frame of an image forming apparatus; a knock-up plate rotatably disposed in the paper feeding cassette and having an upper surface on which the printing mediums are loaded; a knock-up spring applying an elastic force to a lower surface of the knock-up plate to bias the knock-up plate toward the pick-up roller; and a protruding portion disposed on the knock-up plate and facing the pick-up roller, the friction force between the protruding portion and the loaded printing medium is greater than the friction force between the printing mediums.
 17. The paper feeding unit according to claim 16, wherein the protruding portion is formed by protruding the knock-up plate.
 18. The paper feeding unit according to claim 17, wherein the protruding portion is formed by a blanking process of the knock-up plate.
 19. The paper feeding unit according to claim 16, wherein the protruding portion is formed using an outsert molding method.
 20. The paper feeding unit according to claim 16, wherein the protruding portion has a protrusion height less than about 0.2 mm with respect to the upper surface of the knock-up plate.
 21. The paper feeding unit according to claim 20, wherein the protruding portion is formed in a portion of the knock-up plate that is depressed in the opposite direction with respect to the pick-up roller.
 22. The paper feeding unit according to claim 16, wherein at least two protruding portions are arranged on left and right sides of the knock-up plate with respect to a pick-up direction of the printing medium so that a configuration of the protruding portions on the left side is symmetrical to a configuration of the protruding portions on the right side.
 23. The paper feeding unit according to claim 16, wherein the surface of the protruding portion is subject to an embossing process to increase the friction coefficient.
 24. The paper feeding unit according to claim 16, wherein the protruding portion includes one or more squares having a hole in the center thereof.
 25. The paper feeding unit according to claim 24, wherein at least one of the squares having a hole in the center thereof is inclined with respect to a pick-up direction of the printing medium. 